1. Field of the Invention
This invention relates generally to Functionally Gradient Materials (FGMs) and methods for making the same.
2. Description of the Related Art
Functionally Gradient Materials (FGMs) are composites in which the material composition is varied spatially to optimize the performance of the material for a specific application. Since the properties of FGMs also vary spatially, they fall under the category of nonhomogeneous solids. With the introduction of their concept, research into the various aspects of FGMs such as processing, material behavior under different types of loading, fracture mechanics etc., have gained considerable attention and are still being pursued. Most of the investigations which focus on material behavior of FGMs are limited to analytical or numerical studies and very few experimental studies have been reported. One of the major drawbacks associated with experimental studies is the preparation of FGMs having large scale gradation. Preparation of ceramic-metal FGMs, which are used in high temperature applications, is expensive and requires elaborate processing facilities. Therefore, the use of model FGMs to understand the physical phenomena associated with such nonhomogeneous solids is an expedient alternative.
Property gradation in FGMs can be either continuous or in layers. A simple procedure for preparing FGMs graded in layers using polyester resin and plasticizer has been described. V. Parameswaran and A. Shukla, xe2x80x9cDynamic Fracture of a Functionally Gradient Material Having Discrete Property Variationxe2x80x9d, Material Science and Engineering, 33, (1998) 3303-3311. Recently, Marur and Tippur have proposed a gravity assisted casting technique to prepare continuously graded FGMs using epoxy resin and glass beads has been proposed. P. R. Marur and H. V. Tippur, xe2x80x9cEvaluation of Mechanical Properties of Functionally Gradient Materialsxe2x80x9d, To appear in Journal of Testing and Evaluation. (1998). The present invention discloses a continuously graded FGM comprised of polyester, plasticizer and cenospheres and a simple and inexpensive technique to make the same.
Broadly, the invention comprises a continuously graded FGM comprised of polyester, plasticizer and cenospheres. Polyester resin and plasticizer are mixed to form a liquid matrix. Censopheres are added to the matrix. The cenosphere-matrix mixture is then poured into a mold. As the matrix solidifies, the cenospheres, having a lower specific density than the matrix, migrate towards the top of the mold. When the mixture is solid a particulate composite results that has a continuously varying particle volume fraction along a single dimension.
The nonhomogeneous distribution of cenospheres in a polyester matrix is achieved by employing a buoyancy assisted casting process. The overall material properties of the FGM are tailored by adding plasticizer to the polyester matrix. The density, quasi-static and dynamic modulus, quasi-static fracture toughness and dynamic response of the FGMs are obtained as a function of the cenosphere volume fraction. A fractographic analysis of the fractured specimens is also performed to identify the various fracture mechanisms and the results are discussed. The invention further comprises the applicability of some empirical models for estimating the overall properties of the FGM.
The FGM of this invention is lightweight, requires simple processing and is inexpensive. Further, the FGM requires the use of cenospheres which reduces the environmental problems associated with cenosphere disposal. The FGM of this invention can be used in products currently made of polymer resins such as insulation, cabinets, boat hulls and floatation devices.
The FGM of this invention was prepared by mixing a polyester plasticizer with a polyester resin to form a matrix. Cenospheres were added to the matrix and then the matrix-cenospheres mixture was poured into a mold. When the cenospheres-matrix was poured, the top layer of the mixture which is rich in cenospheres is poured first and fills the bottom layer of the mold. Thereafter, the cenospheres diffuse towards the surface of the mold due to buoyancy. When the mixture solidifies, the result is a particulate composite comprised of a nonhomogeneous mixture of censospheres. In the preferred embodiment of the invention, the top layer has the greatest number of cenospheres, the middle layer has an intermediate number of cenospheres and the bottom layer has the least amount of cenospheres thereby resulting in a particulate composite with continuously varying particle volume fraction along a single dimension.
In another embodiment of the invention, a simple procedure for preparing FGMs with continuous gradation of properties was developed using polyester, plasticizer and cenospheres. The FGMs of the invention are characterized by a cenosphere content of the FGMs that increases from  greater than 0 over a distance of 250 mm in a continuous manner, a density that decreases by 20% with increasing cenosphere volume fraction, a quasi static and dynamic modulus that increases linearly with increasing cenosphere volume fraction whereas the compressive strength of the material decreases with increasing cenosphere content and a fracture toughness that increases with increasing cenosphere content whereby a change in fracture mechanism from interface failure to cenosphere breaking was observed at higher cenosphere volume fraction. Further, the estimates of quasi static modulus using the Halpin-Tsai relation with porosity correction matches very well with the measured values and the addition of the plasticizer to the polyester matrix increased the over all fracture toughness and decreased the elastic modulus and compressive strength of the FGMs. The FGMs of the invention are also characterized in that the dynamic peak stress registered by the FGMs decreased with extensive damage to the specimen as the cenosphere content increases.
Because large specimens were prepared, the spatial variation of properties per unit length in this material were small, e.g. the elastic moduli variation was 9 GPa/m.